1. Field of the Invention
The present invention relates to a backup structure for an uprising pipe or downfalling pipe in a vacuum degassing apparatus, more specifically a backup structure for an uprising pipe or downfalling pipe made of platinum or a platinum alloy in a vacuum degassing apparatus. The present invention also relates to a vacuum degassing apparatus and a vacuum degassing method using the above-mentioned backup structure as the backup structure for an uprising pipe or downfalling pipe.
2. Description of the Related Art
FIG. 3 is a cross-sectional view showing a general structure of vacuum degassing apparatuses. The vacuum degassing apparatus 100 shown in FIG. 3 is used to depressurize and degas molten glass G in a melting tank 200 and continuously feed the degassed molten glass to a subsequent treatment bath. The vacuum degassing apparatus 100 shown in FIG. 3 includes a vacuum degassing vessel 102 formed in a cylindrical shape. The vacuum degassing vessel is housed in a vacuum housing 101 so that the longitudinal axis of the vacuum degassing vessel extends in the horizontal direction. The vacuum degassing vessel 102 has an uprising pipe 103 mounted to a lower surface of one end thereof so as to vertically extend and a downfalling pipe 104 mounted to a lower surface of the other end thereof. Each of the uprising pipe 103 and the downfalling pipe 104 is partly housed in the vacuum housing 101. In the vacuum housing 101, the vacuum degassing vessel 102, the uprising pipe 103 and the downfalling pipe 104 are surrounded by a thermal insulating material 107, such as insulating bricks, in order to be covered for thermal insulation.
Each of the uprising pipe and the downfalling pipe of the vacuum degassing apparatus comprises a hollow tube made of platinum or a platinum alloy, such as a platinum/gold alloy or a platinum/rhodium alloy. Since platinum and a platinum alloy are expensive materials, it is desired to make the hollow tube as thin as possible. For this reason, hollow tubes made of platinum or a platinum alloy are usually surrounded by backup structures so that the backup structures give a sufficient mechanical strength to the hollow tubes.
With regard to the backup structure for a conduit used for a molten substance having a high temperature, Patent Document 1 identified below has disclosed a backup structure for an uprising pipe and a downfalling pipe of a vacuum degassing apparatus. Patent Document 1 has disclosed a supporting device for supporting the backup structure for each of the uprising pipe and the downfalling pipe, which includes a supporting plate and a push-up means. The supporting plate is formed in a rectangular shape and is fixed to a lower portion of the uprising pipe. The supporting plate has insulating bricks put thereon so that the insulating bricks are disposed around the uprising pipe so as to cover the uprising pipe.
The push-up means urges the supporting plate upwardly by utilizing the urging force of a coil spring, with the result that the insulating bricks put on the supporting plate are pushed up to bring the top end of the insulating bricks into contact with a pressing plate. This arrangement can support the uprising pipe and the insulating bricks disposed around the uprising pipe. The purpose of the push-up means is to prevent a gap from being formed between adjacent bricks by a thermal expansion difference caused between a platinum uprising pipe or a platinum downfalling pipe and insulating bricks at the time of heating up the vacuum degassing apparatus, for example. If a gap is formed between adjacent bricks, the platinum uprising pipe or the platinum downfalling pipe is broken in some cases. For this reason, a gap is prevented from being formed between adjacent bricks by utilizing the push-up means to push up the bricks to bring the top end of the insulating bricks into contact with the pressing plate.
Patent Document 1 has disclosed that the backup structure has annular flanges (projections) disposed around the uprising pipe at certain intervals h in a vertical direction, and that the bricks are stacked around the uprising pipe so as to sandwich a flange between an upper brick and a lower brick which are adjacent to each other. The elongation of a portion of the uprising pipe between flanges, which are adjacent to each other in the vertical direction, is greater than that of the brick disposed between the adjacent bricks since the platinum uprising pipe has a higher thermal expansion rate than the insulating bricks. The bricks restrain the uprising pipe from elongating in the axial direction thereof, with the result that the uprising pipe is inwardly bent and deformed in a curved shape or a sinuous shape. In this manner, the elongation of the entire uprising pipe in the axial direction becomes equal to the elongation of the stacked bricks. Accordingly, the thermal expansion difference between the uprising pipe and the bricks is distributed in the portions between flanges, which are adjacent to each other in the vertical direction. The amount of bending deformation of the thermally expanded uprising pipe is uniformly distributed in adjacent flanges, with the result that the amount of bending deformation of the uprising pipe is minimized. Patent Document 1 has disclosed that the insulating bricks comprise, e.g., zirconia-based fused cast refractories, and that the zirconia-based fused cast refractories have a corrosion resistance to molten glass G. In the explanation stated above, the reference numerals referred to in Patent Document 1 are used.
Patent Document 1: JP-A-9-059028